In the field of electronic communications, a protection technology is required to ensure stable and reliable operation of communication equipment. For example, a surge protection circuit disposed at a power port or a signal port of the communication equipment can ensure stable and reliable operation of the communication equipment in a severe electromagnetic environment.
Referring to FIG. 1, the prior art provides a surge protection circuit, including a surge protection unit S1 (as shown in a dotted box), a four-row external connection terminal block S2, and a conversion terminal block S3. The surge protection unit S1 includes an internal connection terminal block S11, a first varistor S12, a second varistor S13, a third varistor S14, a fourth varistor S15, and a discharge tube S16. The internal connection terminal block S11 includes a first internal connection terminal L11, a second internal connection terminal L12, a third internal connection terminal L13, and a fourth internal connection terminal N1. One end of the first varistor S12, one end of the second varistor S13, and one end of the third varistor S14 are electrically connected to the first internal connection terminal L11, the second internal connection terminal L12, and the third internal connection terminal L13, respectively; and the other ends of the varistors are connected in parallel together and electrically connected to one end of the fourth varistor S15 and the fourth internal connection terminal N1. The other end of the fourth varistor S15 is electrically connected to one end of the discharge tube S16, and the other end of the discharge tube S16 is grounded. The four-row external connection terminal block S2 includes a first external connection terminal L21, a second external connection terminal L22, a third external connection terminal L23, and a fourth external connection terminal N2. The conversion terminal block S3 includes a first conversion terminal A, a second conversion terminal B, a third conversion terminal C, and a fourth conversion terminal D. The first external connection terminal L21, the second external connection terminal L22, the third external connection terminal L23, and the fourth external connection terminal N2 are electrically connected to the first conversion terminal A, the second conversion terminal B, the third conversion terminal C, and the fourth conversion terminal D, respectively.
Through the surge protection circuit, a 220 V three-phase power distribution manner, a 220 V single-phase power distribution manner, and a 110 V dual live wire power distribution manner can be implemented. In the 220 V three-phase power distribution manner shown in FIG. 1, the first conversion terminal A, the second conversion terminal B, the third conversion terminal C, and the fourth conversion terminal D are electrically connected to the first internal connection terminal L11, the second internal connection terminal L12, the third internal connection terminal L13, and the fourth internal connection terminal N1 through cables, respectively; and then a 220 V phase wire L1, a 220 V phase wire L2, and a 220 V phase wire L3 are electrically connected to the first external connection terminal L21, the second external connection terminal L22, and the third external connection terminal L23, respectively; and a phase wire N is electrically connected to the fourth external connection terminal N2. In the 220 V single-phase power distribution manner shown in FIG. 2, the internal connection terminals L11, L12 and L13 are electrically connected to the conversion terminal A through cables, and the fourth internal connection terminal N1 is electrically connected to the fourth conversion terminal D through a cable. A 220 V phase live wire L is electrically connected to the first external connection terminal L21, and a phase wire N is electrically connected to the fourth external connection terminal N2. In the 110 V dual live wire power distribution manner shown in FIG. 3, the first internal connection terminal L11, the second internal connection terminal L12, and the third internal connection terminal L13 are electrically connected to the second conversion terminal B through cables, and the fourth internal connection terminal N1 is electrically connected to the third conversion terminal C. A 110 V phase live wire L1 and a 110 V phase live wire L2 are electrically connected to the second external connection terminal L22 and the third external connection terminal L23, respectively.
In normal circumstances, for any one of the foregoing power distribution manners, the voltage across the first varistor S12, the second varistor S13, and the third varistor S14 is 220 V or 110 V. The higher the voltage across a varistor is, the smaller the resistance value of the varistor, and the lower the voltage across a varistor is, the larger the resistance value of the varistor is. In this case, resistance values of the first varistor S12, the second varistor S13, and the third varistor S14 are very large. Currents shunted by the first varistor S12, the second varistor S13, and the third varistor S14 from the first internal connection terminal L11, the second internal connection terminal L12, and the third internal connection terminal L13 are nearly zero, and most of currents on the first internal connection terminal L11, the second internal connection terminal L12, and the third internal connection terminal L13 are transmitted to the communication equipment. After the phase wire L1, L2, L3 or L externally connected to the surge protection circuit is struck by lightning, the voltage of the first internal connection terminal L11, the second internal connection terminal L12 or the third internal connection terminal L13 becomes very high, and the voltages across the first varistor S12, the second varistor S13, and the third varistor S14 become very high accordingly. In this case, the resistance values of the first varistor S12, the second varistor S13 and the third varistor S14 become very small. The first varistor S12, the second varistor S13, and the third varistor S14 shunt most of the currents on the first internal connection terminal L11, the second internal connection terminal L12 and the third internal connection terminal L13, respectively, onto the discharge tube S16, and the discharge tube S16 releases the currents to a grounded place. The currents transmitted by the first internal connection terminal L11, the second internal connection terminal L12 and the third internal connection terminal L13 to the communication equipment are small. In this way, a surge protection function is implemented.
During implementation of the present utility model, it is found that the prior art at least has the following problems:
After lightning strike, the currents on the first varistor S12, the second varistor S13, and the third varistor S14 are very large, a parallel current obtained after the three varistors are connected in parallel is greater, it is hard to find a fourth varistor S15 capable of bearing the parallel current. In addition, a residual voltage of the surge protection circuit includes a voltage of the fourth varistor S15 and a voltage of the discharge tube S16, and therefore, the residual voltage of the surge protection circuit is high.